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Fill Your Tank With Citrus

Liquid fuel from an orange? Not just a possibility—a reality, says Agricultural Research Service chemist Karel Grohmann. "For more than 2 years now, we've been making ethanol from citrus waste."

In the early 1940's, citrus waste was dumped on pastures where it fermented and was eaten by cows or worked back into the soil.

But that became impossible with the growth of an industry that currently produces more than 800 thousand dried tons of citrus waste each year. Citrus processors now convert the waste to low-value animal feed, which is not very economically profitable.

Citrus waste consists mostly of sugars. Using enzymes and a strain of E. coli bacterium patented by University of Florida scientists, Grohmann and colleagues have discovered how to convert all the sugars to ethanol and other usable compounds.

"It started as a simple project to make a value-added product from the millions of pounds of waste generated by Florida's citrus juice processing industry," he says.

Grohmann, who heads the U.S. Citrus and Subtropical Products Laboratory in Winter Haven, Florida, worked closely with scientists from the Florida Department of Citrus and the University of Florida to tackle this challenge.

"We knew that yeast ferments sugars to alcohol," he says. "But yeasts work only on 6-carbon sugars, like glucose, fructose, and galactose. Our problem was that the next most abundant sugars found in citrus waste are galacturonic acid, arabinose, and other 5- and 6-carbon sugars that yeasts can't degrade."

More than 90 percent of Florida's citrus crop goes for processing. And half of what goes into a citrus processing plant comes out as a waste product that must be disposed of in an environmentally acceptable way.

"Processors here in Florida add lime to this residue, then press and dry it," Grohmann says. "The press liquor, which contains mostly sugars, is concentrated to citrus molasses that is added to the dried residue and sold for cattle feed."

This processed waste brings only about 3 to 5 cents per pound. And the drying process is energy intensive, meaning it costs producers money.

Citrus waste consists mostly of sugars. Using enzymes and a strain of E. coli bacterium patented by University of Florida scientists, Grohmann and colleagues have discovered how to convert all the sugars to ethanol and other usable compounds.

Working with Bela Buslig, a chemist with the Florida Department of Citrus stationed at the ARS Winter Haven lab, Grohmann began a search for some organism that would ferment not just some of the sugars, but all the compounds in the waste.

They first hydrolyzed orange peel with commercially available enzymes approved by the Food and Drug Administration and commonly used in food processing.

Since the oil in citrus peel naturally inhibits the action of yeast, Grohmann developed a simple nitration system to trap the oil. In it, peel oil adheres to fine particles of wax and is trapped on a filter.

"We filtered out the oil, adjusted pH, and fermented the filtered juice with yeast, breaking down some of the sugars into ethanol," Buslig says.

But the sugars that the yeast cannot convert to ethanol—mainly galacturonic acid—remained, potentially causing another waste disposal problem.

"That's when we introduced the specially modified bacterium," Grohmann says. "Strangely enough, the solubilized citrus pulp contained enough nutrients for the organism to thrive. We didn't need to add anything to the mixture."

In about 48 hours, the bacterium fermented the sugars and galacturonic acid into ethanol, acetic acid, and carbon dioxide—all valuable products. Ethanol is a biofuel, and acetic acid can he used in many food and industrial products such as vinegar, flavor components, and organic solvents. Carbon dioxide can be recaptured to make dry ice.

The bacterium was genetically engineered and patented by Lonnie Ingram and two colleagues from the Department of Microbiology and Cell Science at the University of Florida at Gainesville.

To develop it, "We isolated alcohol-producing genes from an organism that has been in the food chain for centuries and is found in plant saps and honey," Ingram says, "We put these genes into strains of E. coli bacteria that are prevalent in nature, and we redirected their digestive systems." The work was supported by USDA and the Department of Energy.

Using Ingrain's organism, called E. coli KO11, Grohmann and colleagues were the first to convert galacturonic acid to ethanol.

According to W. Lamar Harris, ARS' national program leader for energy, this work is especially timely, since ongoing research to convert corn to fuel has dramatically increased fuel ethanol production.

From virtually none in the early 1970's, commercial ethanol production reached 1.1 billion gallons in 1994 and is expected to increase to meet the demand for oxygenate for reformulated gasolines.

For several years, scientists have been turning corn sugars into fuel. Harris says that "increasing ethanol production and capacity will expand markets for corn producers and provide economic opportunities for rural America."

The question is: Could Grohmann's research do the same for citrus and other fruit processing wastes? -- By Doris Stanley, ARS.

USDA-ARS Citrus and Subtropical Products Research Laboratory, 600 Avenue S, Northwest, Winter Haven, FL 33881; phone (863) 293-4133

"Fill Your Tank With Citrus" was published in the June 1995 issue of Agricultural Research magazine.